The present invention relates to satellite-navigation receivers and systems, and more particularly to improvements to the radio sensitivity of GPS receivers that help continue to provide position solutions in indoor and other covered locations.
Global positioning system (GPS) receivers use signals received from typically three or more earth-orbiting satellites to determine navigational data such as position and velocity. GPS signals are available worldwide at no cost and are now being routinely used to determine the location of automobiles to within one city block, or better. Dual-frequency carrier GPS receivers typically track a pair of radio carriers, L1 and L2, associated with the GPS satellites to generate accumulated delta-range measurements (ADR) from P-code modulation on those carrier frequencies and at the same time track L1 C/A-code to generate code phase measurements. Carrier frequency L1 is allocated to 1575.42 MHz and carrier frequency L2 is positioned at 1227.78 MHz. Less expensive receivers tune only one carrier frequency, and therefore do not have adequate information to compute the local troposheric and ionospheric signal-propagation delays that appear as position errors. At such frequencies, radio carrier signals travel by line-of-sight. Thus buildings, mountains and the horizon can block reception, and multipath reflections can interfere with good reception.
Each one of the constellation of GPS satellites in orbit about the earth transmits one of thirty-two unique identifying codes in a code-division multiple access (CDMA) arrangement. Such allows all of the many GPS satellites to transmit in spread spectrum mode at the same frequency, plus or minus a Doppler frequency shift of that frequency as results from the satellite""s relative velocity. Particular satellites are sorted out of a resulting jumble of signals and noise by correlating a 1023 xe2x80x9cchipxe2x80x9d code to one of the thirty-two pseudo random number (PRN) sequence codes that are preassigned to individual GPS satellites. These codes are not necessarily being transmitted in phase with one another. Therefore, xe2x80x9cfindingxe2x80x9d a GPS satellite initially involves searching various carrier frequencies, to account for Doppler frequency shift and local crystal oscillator inaccuracies. The searching also needs to find a code match, using 1023 different code phases and twenty or more possible correlation code templates.
The single largest uncertainty stems from the random frequencies possible from typical local oscillators at start-up. Therefore, the apparent-Doppler frequency is known only within wide search boundaries. Knowing the actual Doppler frequency is not much help, because the local oscillator can be so far off nominal on its own.
From the user""s standpoint, at least two operational characteristics of prior art GPS receivers interfere with complete satisfaction. Such conventional receivers often quit working indoors because the buildings reduce the local signal field level to less than the receiver""s maximum sensitivity. And, most receivers take a very long time to produce a position solution from a cold start.
Indoors, the signal-to-noise ratio (SNR) typically drops too low to provide a useful signal for conventional GPS receivers. The available signal gets buried in the noise. In general, two methods can be used to improve a receiver""s SNR, coherent sample averaging and non-coherent sample averaging. In the coherent method, the analog-to-digital samples are summed before squaring, e.g.,             ∑      m        ⁢    P    ,
where   P  =                    (                              ∑            n                    ⁢          I                )            2        +                  (                              ∑            n                    ⁢          Q                )            2      
and I and Q are summed over ten milliseconds. In the non-coherent method, the samples are summed after squaring.
The objective in such averaging is to reduce the statistical variance. Since the SNR is defined as       10    ⁢          xe2x80x83        ⁢    log    ⁢          xe2x80x83        ⁢                  A        2                    2        ⁢                  xe2x80x83                ⁢                  σ          2                      ,
reducing "sgr", the variance of uncorrelated noise, will improve the SNR and therefore the receiver sensitivity. In the coherent averaging method, the improvement is 10 log N, where N is the number of milliseconds being summed up. In the non-coherent averaging method, the improvement is 10 log{square root over (M)}, where M is the number of milliseconds being summed up. Together, the overall improvement in a prior art receiver could be 10 log N+10 log{square root over (M)}. But too large an N can increase integration times and narrow the frequency response. The narrower the frequency response is, the longer it will take for the receiver to find initial lock with the carrier because finer steps must be taken in the search. So the gains in receiver sensitivity can come at the sacrifice of reasonable time-to-first-position-fix performance.
It is therefore an object of the present invention to provide a satellite-navigation receiver that can work indoors with extremely low signal-strength levels.
It is another object of the present invention to provide a satellite-navigation receiver that produces position solutions rapidly after each cold start.
It is a further object of the present invention to provide a satellite-navigation system that is inexpensive.
Briefly, a receiver embodiment of the present invention comprises a digital sampler that precedes digital signal processing which can operate at a high rate and a low rate. If the high rate is selected for noise reduction by non-coherent averaging, the samples are averaged over time and transformed to the low rate. The digital signal processor is fed only low-rate samples in either case.
An advantage of the present invention is that a system and method are provided that substantially increase the sensitivity of navigation receivers.
Another advantage of the present invention is that a system and method are provided that improve sensitivity and time-to-first-fix enough for urban canyon and indoor use.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.